The Jedi Order is the light side of the Force, or the good guys, and they dedicate their lives to the attainment of peace while rocking robes in calming desert tones. The Sith Order is the dark side of the Force (the bad guys), and they choose to use their powers for evil, personal use, since they believe the dark side is much stronger and usually dress like futuristic Dementors. As the Jedi strive for calm tranquility to maintain inner and societal peace, the Sith power comes from a self-destructive passion, which leads to very different goals than what the Jedi Order would find acceptable. This is why Yoda famously warns, "Fear leads to anger, anger leads to hate, hate leads to suffering."
